This invention generally relates to the deposition of material onto a substrate, especially in the field of microelectronic circuit production. The invention especially relates to the deposition of metals or dielectric substances onto the surface of semiconductors. In addition, this invention relates to the resolution of patterns formed by deposition of a material onto a substrate.
In the art of microelectronic circuitry, it is often necessary for layers and/or patterns to be formed on the surface of a semiconductor substrate by deposition, oxidation or epitaxial growth. For example, such processes are involved in providing semiconductors with ohmic contacts, Schottky barriers and dielectric barriers, all of which are required in the production of complicated integrated circuits. For the deposition of metals, metal alloys, or dielectric substances, the conventional processes require numerous steps, including: etching; photoresist deposition, irradiation and development; metallization; and annealing. In the production of complicated circuits, each of these steps must be repeated several times with each successively deposited layer being applied so as to register with previous layers with sub-micron accuracy.
Under current procedures for the deposition of a metal or a dielectric substance, photolithographic processes are usually employed such as disclosed in U.S. Pat. Nos. 4,349,408 (Tarng et al) and 4,568,411 (Martin). Initially, the substrate wafer is cleaned and etched if necessary. A photoresist is then spun onto the wafer and exposed to irradiation through a carefully aligned mask. The type of photoresist used is chosen on the basis of the desired primary processing step which is to follow, e.g., etching or metallization.
In order to achieve good resolution and edge definition of the photoresist relief structure, it is quite often required that the mask be pressed firmly against the photoresist during irradiation. For this reason, it is important that the front and back surfaces of the wafer be flat and parallel. Otherwise, edge definition of the resultant relief structure will be poor. Furthermore, if the front and back surfaces of the wafer are not flat and parallel, substrate damage such as breakage may occur.
After the mask is brought into position, the photoresist is exposed through the mask by visible, ultraviolet, e-beam, or X-ray radiation. The choice of radiation is dependent upon the type of photoresist and the resolution and nature of the patterns required for the resultant relief structure. Normally, the shorter the wavelength of exposure radiation, the better the resolution of the desired pattern. The unexposed resist (negative resist) or exposed resist (positive resist) is removed by solvent and the surface of the substrate is cleaned. Frequently, the remaining resist, which forms the relief structure on the substrate, requires the further step of thermal annealing for good adhesion.
After exposure and development of the photoresist, the wafer is subjected to the primary processing step such as etching, deposition of a metal or dielectric, or both.
Following the primary processing step, the initial photoresist relief structure is removed and the next photoresist layer is spun onto the wafer to form a new relief structure for use in the next metallization or etching step. The successive photoresist layers must be applied in register to sub-micron degree and the chemistry of the photoresist layers as well as their topography must be suitable for the intended primary processing step whether it be etching, metallization, or some other processing step.
During the cycling of the substrate wafer through the successive steps of photoresist deposition, photoresist exposure and development, followed by etching and/or material deposition, the risk of modifying the surface chemistry of the substrate wafer in some fashion, which in almost all circumstances is an undesirable result, remains very high. Furthermore, often times complete removal of the photoresist layer requires etching the semiconductor surface, resulting in an unwanted removal of material from the surface.
Another disadvantage of the prior art multistep photolithographic process is that for the production of a microelectronic circuit requiring several metallizations and/or etching steps, the excessive amount of handling of the substrate involved leads to an increased risk of damage to the substrate, e.g., breakage, bending or contamination.
Another disadvantage associated with the prior art processes is that during material deposition steps, such as chemical vapor deposition of dielectrics or evaporation or sputtering techniques used in metallization, the entire wafer is often subjected to high temperatures which may also detrimentally affect the surface chemistry of the semiconductor.
Furthermore, the conventional deposition techniques are limited in regard to the types of materials which can be successfully deposited on a substrate wafer. In evaporation processes, a metal charge is evaporated and then deposited on a cold substrate and photoresist by physical vapor deposition. This procedure is suitable for atomically pure metals. However, when the metal charge is an alloy, it is not always possible to control evaporation so that the components of the alloy will concurrently evaporate. This renders it difficult to achieve the desired stoichiometry within the deposited alloy layer.
In the metallization technique known as sputtering a target material is bombarded by ions whereby surface atoms of the target material become volatile and are deposited by physical vapor deposition onto the substrate and photoresist. However, here again not all deposit materials are suitable for the sputtering technique. Moreover, this procedure tends to degrade the definition of the photoresist relief structure and also may damage the surface of the wafer itself.
The processes of evaporation and sputtering are not well suited for the deposition of refraction materials such as tungsten or dielectrics, e.g., SiO.sub.2. For these materials, chemical rather than physical vapor deposition is commonly used which requires precise control of reaction conditions.
Another process for metal deposition is disclosed in U.S. Pat. No. 4,511,600 (Leas), wherein a metal pattern is deposited on a substrate by forcing a molten metal through a vibrated array of orifices.
In recent years, lasers have been used in conjunction with conventional primary processing steps such as deposition, plating and etching. For example, the use of lasers in chemical vapor deposition steps are disclosed by Baum et al, "Laser Chemical Vapor Deposition," Appl. Phys. Lett. 47 (6), Sept., 1985, p. 538-40 and Higashi et al, "Patterned Aluminum Growth via Exterior Laser Activated Metalorganic Chemical Vapor Deposition," Appl. Phys. Lett. 48 (16), April 1986, pp. 151-53. Laser-enhanced plating processes are disclosed by Gutfeld et al, "Laser-Enhanced Plating and Etching Mechanisms and Applications," IBM J. Res. Develop., Vol. 24:No.2, March 1982, pp. 136-44 and Gutfeld et al, "Recent Advances in Laser-Enhanced Plating", Mol. Res. Soc. Symp. Proc., Vol. 29, 1984, pp. 325-32.
Lasers have also been used in the decomposition of thin films coated on substrates. See Fisanick et al, "Relation of Local Transformation Dynamics to Final Reaction Profiles in Laser-Initiated Decompositions of Thin Films", pp. 157-59; Gross et al, "Laser-Initiated Deposition Reactions: Microchemistry in Organogold Polymer Films", Appl. Phys. Lett. 47(9) November, 1985, p. 923 and U.S. Pat. No. 4,526,807 (Auerbach).
In addition, lasers have been used for injecting particles into substrates (U.S. Pat. No. 4,299,860, Schaefer et al); in printing of print compounds encapsulated in polyarylene sulfide substrates (U.S. Pat. No. 4,560,580, Needham et al); and in surface alloying (U.S. Pat. No. 4,495,255, Draper et al).